This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 of Appln. No. 100 40 869.9 filed in Germany on Aug. 21, 2000, the entire content of which is hereby incorporated by reference.
The invention relates to a method for the fluid-mechanical stabilization of a premix burner, into which a combustion air stream is fed tangentially into an interior burner chamber, is mixed with an injected gaseous and/or liquid fuel while forming a coaxially oriented swirl flow and induces a reverse flow zone at a burner outlet that is used during the operation of the burner to stabilize the flame. The invention furthermore relates to a premix burner for performing the method. A preferred field of application of the invention is the operation of a gas turbine system.
Premix burners of the type discussed here are known from EP 0 321 809 and EP 0 780 629. Such burners, characterized by very low noxious emissions, are used widely in combustors of gas turbine systems for hot gas generation.
When operating gas turbine systems, thermoacoustic oscillations often occur in the combustors. Fluid-mechanical instability waves created at the burner result in the formation of flow vortices that have a major effect on the entire combustion process and result in undesirable, periodic heat releases within the combustor that are associated with major fluctuations in pressure. The high fluctuations in pressure are coupled with high oscillation amplitudes that can lead to undesirable effects, such as, for example, a high mechanical load on the combustor housing, increased NOx emissions caused by inhomogeneous combustion, and even an extinction of the flame within the combustor.
Thermoacoustic oscillations are based at least in part on flow instabilities of the burner flow that express themselves as coherent flow structures and influence the mixing processes between air and fuel. In standard combustors, cooling air is passed in the form of a cooling air film over the combustor walls. In addition to the cooling effect, the cooling air film also has a sound-dampening effect and helps to reduce thermoacoustic oscillations. In modern high-efficiency gas turbine combustors with low emissions and constant temperature distribution at the turbine inlet, the cooling air flow into the combustor is clearly reduced, and the entire air is passed through the burner. However, at the same time the sound-dampening cooling air film is reduced, causing a reduction in the sound-dampening effect so that there is once again an increase in the problems associated with undesirable oscillations.
Another possibility for dampening the sound is the connection of so-called Helmholtz resonators near the combustor or cooling air supply. However, because of tight space conditions, it is very difficult to provide such Helmholtz resonators in modern combustion chamber designs.
It is also known that the fluidic instabilities and associated pressure fluctuations occurring in the burner can be countered by stabilizing the fuel flame with an additional injection of fuel. Such an injection of additional fuel is performed through the head stage of the burner that is provided with a jet for the pilot fuel gas supply located on the burner axis; however, this results in an over-rich central flame stabilization zone. This method of reducing thermoacoustic oscillation amplitudes has the disadvantage, however, that the injection of fuel at the head stage may be associated with increased NOx emissions.
Closer studies regarding the formation of thermoacoustic oscillations have shown that such undesirable, coherent structures are formed during mixing processes. Of special significance hereby are the shear layers forming between two mixing flows, which are formed within the coherent structures. More detailed explanations regarding this phenomenon can be found in the following publications: Oster and Wygnansky, 1982, xe2x80x9cThe forced mixing layer between parallel streams,xe2x80x9d Journal of Fluid Mechanics, Vol. 123, 91-130; Paschereit et al., 1995, xe2x80x9cExperimental investigation of subharmonic resonance in an axisymmetric jet,xe2x80x9d Journal of Fluid Mechanics, Vol. 283, 365-407.
As can be seen from the previous articles, it is possible to influence the coherent structures forming within the shear layers by introducing an acoustic excitation in a targeted manner in such a way that their formation is prevented. Another method is the introduction of an acoustic anti-sound field so that the existing, undesired sound field is properly extinguished by a phase-shifted sound field that has been introduced in a targeted manner. The anti-sound technique, as described, requires a relatively high amount of energy, however, which must be provided to the burner system either externally or must be branched off from the entire system at a different point, which, however, will result in a small, yet existing, loss of efficiency.
In addition to the previous active methods for specifically reducing the coherent structures forming inside burners, such interferences in the burner flow alternatively can be countered with passive measures. Passive measures, i.e., primarily constructive design characteristics of the burners, that extend the operating range of a burner with respect to pulsations and emissions are especially attractive, since, once installed, they do not require any additional maintenance.
The invention is based on the objective of creating a method for increasing the fluidic stability of a premix burner, which efficiently and without further energy consumption suppresses the undesired flow eddies that form as coherent pressure fluctuation structures. The measures necessary on a premix burner for this purpose should be simple to construct and cheap to realize. The measures used also should be completely maintenance-free.
According to the invention, the objective is realized with a method for increasing the fluidic stability of a premix burner as well as with a premix burner of the type mentioned in the independent claims. Characteristics that constitute advantageous further development of the concept of the invention are described in the dependent claims and the specification, as well as in the exemplary embodiments.
The method according to the invention is based on the basic idea ofxe2x80x94for the fluid-mechanical stabilization of a premix burner, into which at least one combustion air stream is fed tangentially into a burner chamber and is mixed with an injected gaseous and/or liquid fuel while forming a swirl flow oriented coaxially to the burner axis and induces a reverse flow zone at a change in the cross-section on a burner mouth, that is used during the operation of the burner to stabilize the flamexe2x80x94increasingly, radially deforming the swirl flow within the burner chamber in the direction of the burner mouth towards at least one circumferential section and let it enter the combustor in a non-rotation-symmetrical flow cross-section, whereby this deformation is created by reducing the free flow cross-section of the burner chamber.
The formation of coherent vortex structures is hindered by a shape of the flow cross-section that deviates from the rotation symmetry in the burner chamber and on entering the combustor. In premix burners according to the state of the art, the time delay of the fuel from the injection point to the flame is constant at certain operating points. The deformation of the flow-cross-section according to the invention results in a broad distribution of the delay time. The prevention of the formation of vortex structures at the burner outlet and a smudged time delay also suppresses a periodic heat release, which again is responsible for the occurrence of thermoacoustic oscillations. By forcing the deformation of the swirl flow through constricting sections of the chamber contour, as will still be explained at another place, this results in an acceleration of the flow, which acts in a stabilizing manner on the reverse flow zone.
A premix burner according to the invention is based on a premix burner for use in a heat generator, comprising essentially a swirl flow generator with means for the tangential introduction of at least one gaseous and/or liquid fuel into the combustion air stream with concomitant formation of a swirl flow with an axial movement component up to the burner mouth, at which the swirl flow bursts while inducing a reverse flow zone. A burner according to this type, based on at least two hollow, conically expanding partial bodies stacked inside each other in the flow direction of the hot gases, the center axes of which extend offset to each other, is described in EP 0321809, which is an integrated part of this application. Such burner types, also called cone burners or double cone burners, are provided at their burner outlet with a separation edge, whose edge contour consists of two semi-circles, offset from each other, but whose edge contour is almost circular and therefore approximately rotation-symmetrical to the burner axis when closed. The fuel/gas mixture forming inside the burner chamber spreads in the form of a rotation-symmetrical swirl flow with an axial component towards the burner mouth, with all its known disadvantages with respect to the formation of coherent structures and associated thermoacoustic pressure fluctuations.
If, however, it is ensured that targeted radial deformations are introduced into the flow of the fuel/air mixture in such a way that the flow cross-section differs from that of an rotation-symmetrical flow, the formation of coherent structures and a constant time-delay of the fuel can be effectively countered in this way.
Such an influencing of the flow geometry can be achieved with at least one section of the chamber wall, where said wall section has a smaller slope in a down-stream end part of the burner chamber than in an upstream part. In this way, this at least one section, in contrast to those wall sections at the same axis level that do not possess this property, results in a radial deviation from the circular shape in the direction towards the burner axis. Any partial non-circular contours in the burner chamber and at the outlet edge, for example, straight or spherically curved edge sections along the circumference, help in reducing flow vortices.
It should be observed as a principal design rule for designing the burner outlet edge that the geometrical deviation from a circular geometry should be chosen at least so large that the resulting distance between the two geometries is greater than the boundary layer thickness of the flow that flows through the outlet geometry.